This is mostly by design; returning error messages in pages can be a security hole in some cases. (Yes, I treat the server as though it were being used seriously, and it is quite paranoid about malformed inputs and such.) If you want debugging information, try out the technique implemented in bug 540633 (speaking of which, that should be documented -- I'll poke zwol about it when he gets back from PTO) -- or tweak testing/mochitest/server.js by adding |DEBUG = true;| to it.